This invention relates to a power supply circuit comprising:
a transformer having a primary winding connected to receive a primary voltage and having a secondary winding for generating a secondary voltage; a load device and a rectifier diode connected in series across the secondary winding to supply a current to the load device; and an on/off switch for interrupting the current from the secondary winding to the load device.
Such a power supply circuit is used in portable mains-powered equipment, which may or may not comprise rechargeable batteries. The transformer provides the electrical isolation from the mains, which is required for safety reasons. Particularly in portable equipment comprising rechargeable batteries there is an increasing trend to use switched-mode power supplies instead of the customary 50/60 Hz transformer, in order to raise the efficiency and, especially, to reduce the weight of the power supply. A further development is the separation between the power supply and the actual apparatus with the batteries. There is a trend towards a "power plug", which incorporates a switched-mode power supply and the transformer and which supplies a non-hazardous electrically isolated low voltage for use in the apparatus to be coupled to the power plug. The apparatus accommodates the other power supply components including rechargeable batteries, if present. Since the apparatus itself is isolated from the mains voltage, regulations for approval are less severe or are not imposed at all. This also applies to the on/off switch of the apparatus if this switch has been included in the secondary circuit of the transformer as shown in FIG. 1, which represents a known configuration in which an on/off circuit has been included in the secondary circuit formed by the load device and the rectifier diode. The on/off switch can be a normal hand-operated switch. Alternatively, the on/off switch can be an electronic switch which, as is known from Patenschrift DE 43 39 160 C1, in addition to or instead of the control means provided in the power plug, can be switched on and off by the load device in such a rhythm, duty cycle control, that the average current is given the desired value, for example, to charge the batteries. The electronic series switch should be capable of handling large currents, particularly in apparatuses with rechargeable batteries, and dissipates power. Since the electronic switch is turned on by a control signal from the load device, the control signal will be small or absent in the case of empty batteries and the electronic series switch can no longer be turned on at the instant at which the power plug is connected to the mains voltage. Special measures must be taken to preclude this latch-up situation and this requires more electronic circuitry.